Method of cleaning wafer after CMP

ABSTRACT

A method includes forming a first dielectric layer over a wafer, etching the first dielectric layer to form an opening, filling a tungsten-containing material into the opening, and performing a Chemical Mechanical Polish (CMP) on the wafer. After the CMP, a cleaning is performed on the wafer using a weak base solution.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of the following provisionally filedU.S. patent application: Application Ser. No. 62/427,336, filed Nov. 29,2016, and entitled “Method of Cleaning Wafer after CMP;” whichapplication is hereby incorporated herein by reference.

BACKGROUND

In the manufacturing of integrated circuits, contact plugs are used forconnecting to the source and drain regions and the gates of transistors.The source/drain contact plugs were typically connected to source/drainsilicide regions, which are formed by forming contact openings to exposesource/drain regions, depositing a metal layer, performing an anneal toreact the metal layer with the source/drain regions, filling tungsteninto the remaining contact opening, and performing a Chemical MechanicalPolish (CMP) to remove excess tungsten. A cleaning is then performed.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 12 are cross-sectional views of intermediate stages inthe formation of a transistor and contact plugs in accordance with someembodiments.

FIG. 13 illustrates a cross-sectional view of a contact plug with acarbon-rich layer left on top of a contact plug.

FIG. 14 illustrates the cleaning performed on an additional metallicfeature using an acid solution or a neutral liquid.

FIG. 15 illustrates a process flow for forming a transistor inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A transistor having contact plugs connected to silicide regions and themethod of forming the same are provided in accordance with variousexemplary embodiments. The intermediate stages of forming the transistorare illustrated. The variations of some embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements. The steps shownin FIGS. 1 through 12 are also illustrated schematically in the processflow 200 as shown in FIG. 15.

FIGS. 1 through 12 are cross-sectional views of intermediate stages inthe formation of a transistor and the respective contact plugs inaccordance with some exemplary embodiments. Referring to FIG. 1, wafer10 is provided. Wafer 10 includes substrate 20, which may be formed of asemiconductor material such as silicon, silicon germanium, siliconcarbon, a III-V compound semiconductor material, or the like. Substrate20 may be a bulk substrate or a Semiconductor-On-Insulator (SOI)substrate.

Gate stacks 26A and 26B, which are collectively referred to as gatestacks 26, are formed over substrate 20. In accordance with someembodiments of the present disclosure, gate stacks 26A and 26B areformed as gate stack strips (in a top view of wafer 10) havinglengthwise directions parallel to each other, wherein the distancebetween gate stacks 26A and 26B is minimized. Each of gate stacks 26Aand 26B may include gate dielectric 24, gate electrode 28 over gatedielectric 24, and hard mask 38 over gate electrode 28. In accordancewith some embodiments of the present disclosure, gate stacks 26 arereplacement gate stacks, which are formed by forming dummy gate stacks(not shown), removing the dummy gate stacks to form recesses, andforming the replacement gates in the recesses. As a result, each of gatedielectrics 24 includes a bottom portion underlying the respective gateelectrode 28, and sidewall portions on the sidewalls of the respectivegate electrode 28. The sidewall portions form rings encircling therespective gate electrodes 28.

In accordance with some embodiments of the present disclosure, sourceand drain regions 22 (referred to as source/drain regions 22hereinafter) are formed to extend into substrate 20. In accordance withalternative embodiments, source/drain regions 22 are formed after theformation of contact opening as shown in FIG. 2. One or more ofsource/drain regions 22 may be a common source region or a common drainregion shared by gate stacks 26A and 26B. Accordingly, gate stack 26Amay form a first transistor in combination with the source/drain regions22 on the opposite sides of gate stack 26A, and gate stack 26B may forma second transistor in combination with the source/drain regions 22 onthe opposite sides of gate stack 26B. The first transistor and thesecond transistor may be electrically connected in parallel to act as asingle transistor.

Gate dielectric 24 may be a single layer or a composite layer thatincludes a plurality of layers. For example, gate dielectric 24 mayinclude an interfacial oxide layer and a high-k dielectric layer overthe oxide layer. The oxide layer may be a silicon oxide layer formedthrough thermal oxidation or chemical oxidation. The high-k dielectriclayer may have a k value greater than 7, or even greater than 20.Exemplary high-k dielectric materials include hafnium oxide, zirconiumoxide, lanthanum oxide, and the like.

In accordance with some embodiments of the present disclosure, each gateelectrode 28 has a single-layer structure formed of a homogeneousconductive material. In accordance with alternative embodiments, eachgate electrode 28 has a composite structure including a plurality oflayers formed of TiN, TaSiN, WN, TiAl, TiAlN, TaC, TaN, aluminum, oralloys thereof. The formation of gate electrodes 28 may include PhysicalVapor Deposition (PVD), Metal-Organic Chemical Vapor Deposition (MOCVD),and/or other applicable methods. Hard masks 38 may be formed of siliconnitride, for example.

In accordance with alternative embodiments of the present disclosure,rather than being replacement gate stacks, gate stacks 26A and 26B areformed by depositing a blanket gate dielectric layer and a blanket gateelectrode layer (such as a polysilicon layer), and then patterning theblanket gate dielectric layer and the blanket gate electrode layer.

Referring again to FIG. 1, Contact Etch Stop Layer (CESL) 34 is formedto cover substrate 20, and may extend on the sidewalls of gate spacers30. In accordance with some embodiments of the present disclosure, CESL34 is formed of silicon nitride, silicon carbide, or other dielectricmaterials. Inter-Layer Dielectric (ILD) 36 is formed over CESL and gatestacks 26A and 26B. ILD 36 may be formed of an oxide such asPhospho-Silicate Glass (PSG), Boro-Silicate Glass (BSG), Boron-DopedPhospho-Silicate Glass (BPSG), Tetra Ethyl Ortho Silicate (TEOS) oxide,or the like. The formation may include, for example, Chemical VaporDeposition (CVD), Flowable CVD (FCVD), spin-on coating, or the like.

Referring to FIG. 2, ILD 36 and CESL 34 are etched to form contactopening 40. The respective step is illustrated as step 202 in theprocess flow shown in FIG. 15. Opening 40 is a source/drain contactopening in accordance with some embodiments. Source/drain region 22 (ifalready formed) is exposed to contact opening 40. In accordance withsome embodiments of the present disclosure, opening 40 has width W1smaller than about 40 nm. Depth D1 may be greater than about 45 nm.

In accordance with some embodiments in which source/drain regions 22have not been formed yet at this time, a Pre-Amorphization Implantation(PAI) and a source/drain implantation may be performed to formsource/drain regions 22, wherein the species of the PAI and theimplanted impurity for forming source/drain regions 22 are implantedinto substrate 20 through opening 40. The PAI may be performed usinggermanium, silicon, or the like, which destroys the lattice structure ofthe implanted regions in order to control the depth of the subsequentsource/drain implantation. The source/drain implantation may beperformed using boron or indium if the respective transistor is a p-typetransistor, or using phosphorous, arsenic, or antimony if the respectivetransistor is an n-type transistor.

FIG. 3 illustrates the formation of contact (plug) spacers 44 inaccordance with some embodiments of the present disclosure. Theformation of contact spacers 44 may include depositing one or aplurality of conformal dielectric layer(s). The dielectric layer extendinto contact opening 40, and includes vertical portions on the sidewallsof ILD 36, and horizontal portions at the bottom of opening 40 as wellas over ILD 36. The deposition process is performed using a conformaldeposition process such as Atomic Layer Deposition (ALD), CVD, or thelike, so that the horizontal portions and vertical portions of thedeposited layer have similar thicknesses. An anisotropic etching is thenperformed to remove the horizontal portions, leaving the verticalportions as contact spacers 44. The anisotropic etching may be performedusing ammonia (NH₃) and NF₃ as etching gases. It is noted that contactspacers 44 in the same opening 40, when viewed in a top view of wafer10, are portions of an integrated spacer ring.

In accordance with some embodiments of the present disclosure, spacers44 are formed of a dielectric material that has a high etchingselectivity relative to oxide, so that in subsequent cleaning processes(in which oxides are removed), the spacers are not damaged. For example,contact spacers 44 may be formed of silicon nitride, siliconoxy-carbide, silicon oxynitride, or the like.

In accordance with alternative embodiments of the present disclosure,spacers 44 are not formed. Accordingly, spacers 44 are illustrated usingdashed lines to indicate this step may be performed or skipped. Inaccordance with these embodiments, the subsequently formed metal layer46 (FIG. 4) may have sidewall portions in contact with the sidewalls ofILD 36.

Next, referring to FIG. 4, metal layer 46 is deposited. The respectivestep is illustrated as step 204 in the process flow shown in FIG. 15. Inaccordance with some embodiments of the present disclosure, metal layer46 is a titanium (Ti) layer, which may be formed using Physical VaporDeposition (PVD). Metal layer 46 includes bottom portion 46A at thebottom of opening 40, and sidewall portions 46B on the sidewall surfacesof ILD 36. Metal layer 46 has two functions. The first function is thatthe bottom portion of metal layer 46 reacts with the underlyingsource/drain region 22 to form a source/drain silicide region. Thesecond function is that metal layer 46 acts as an adhesion layer for thesubsequently formed capping/adhesion layer.

Referring to FIG. 5, capping layer 48 is deposited. The respective stepis illustrated as step 206 in the process flow shown in FIG. 15. Cappinglayer 48 also acts as a diffusion barrier layer. In accordance with someembodiments of the present disclosure, capping layer 48 is formed of ametal nitride such as titanium nitride. Capping layer 48 may be formedusing PVD, CVD, or the like.

FIG. 6 illustrates a silicidation process for forming silicide region50. In accordance with some embodiments of the present disclosure, thesilicidation process is performed through an anneal, which isrepresented by arrows 52. The respective step is illustrated as step 208in the process flow shown in FIG. 15. The anneal may be performedthrough Rapid Thermal Anneal (RTA), furnace anneal, or the like.Accordingly, the bottom portion of metal layer 46 reacts withsource/drain region 22 to form silicide region 50. The sidewall portionsof metal layer 46 remain after the silicidation process, as shown inFIG. 6. In accordance with some embodiments of the present disclosure,the bottom portion of metal layer 46 is fully reacted, and the topsurface of silicide region 50 is in contact with the bottom surface ofcapping layer 48.

Next, metallic material 54 is filled into the remaining contact opening40, and the resulting wafer 10 is shown in FIG. 7. The respective stepis illustrated as step 210 in the process flow shown in FIG. 15.Metallic material 54 may be formed of a tungsten-containing material,which may be formed of pure or substantially pure tungsten (for example,with an atomic percentage greater than about 95 percent). In accordancewith alternative embodiments of the present disclosure, metallicmaterial 54 comprises titanium. Metallic material 54 is free fromcopper, cobalt, or the like since these materials will not benefit fromthe embodiments of the present disclosure due to different behaviorsthan tungsten in the subsequent process steps.

Next, as also shown in FIG. 7, a planarization process such as aChemical Mechanical Polish (CMP) is performed to remove the excessportions of metallic material 54, capping layer 48, and metal layer 46over ILD 36. Source/drain contact plug 56 is thus formed, as shown inFIG. 8. The respective step is illustrated as step 212 in the processflow shown in FIG. 15. FIG. 7 schematically illustrates polish pad 57.In an actually CMP process, polish pad 57 may have a size/diametergreater than the size of wafer 10. During the CMP process, polish pad 57may face up, while wafer 10 may face down and is pressed against polishpad 57. Wafer 10 is rotated during the CMP. A slurry (not shown) isdispensed on polish pad 57 during the CMP.

After the CMP is finished, wafer 10 is cleaned in a cleaning step. Therespective step is illustrated as step 214 in the process flow shown inFIG. 15. FIG. 8 schematically illustrates an exemplary cleaning process.Brush 61 may be used to brush wafer 10 during the cleaning to scrub offthe fall-on, slurry, and the by-products generated during the CMP. Inaccordance with some embodiments of the present disclosure, brush 61 isa roller sponge brush, which rotates during the cleaning as illustrated.Brush 61 may also be a pencil brush. In an exemplary cleaning, a rollersponge brush may be used to brush wafer 10 first, followed by thecleaning using a pencil brush. When brush 61 is used to brush wafer 10,cleaning solution 58 is also applied on wafer 10, for example, throughspray, so that the fall-on, slurry, and the by-products generated duringthe CMP may be removed. Brush 61 is in physical contact with wafer 10during the cleaning. The cleaning may be performed for a period of timebetween about 1.5 minutes and about 2.5 minutes, with cleaning solution58 being at a temperature between about 18° C. and about 21° C.

The cleaning environment may result in a galvanic electrode includingcontact plug 56 and the parts of wafer 10 electrically connected tocontact plug 56, especially for NMOS transistors and N-Well regions.Accordingly, electrons may be conducted away from contact plug 56 intosource/drain region 22. The metal such as tungsten, with the electronslost to substrate 20, thus becomes metal ions with positive charges. Themetal ions may be lost into the cleaning solution 58 in the resultingcorrosion, resulting in metal loss and the recessing of contact plug 56.With the recessing, the top surface of contact plug 56 will be lowerthan the top surface of ILD 36. In the embodiments in which the width W2of contact plug 56 is very small, for example, smaller than about 20 nm,the resulting recess is deep and narrow (with a high aspect ratio), andit is difficult to fully fill the recess with the subsequently formedoverlying conductive features such as via or contact plug 72 as shown inFIG. 12. In accordance with some embodiments of the present disclosure,an inhibitor is used in cleaning solution 58 to reduce, and possiblyeliminate, the recessing of contact plug 56 during the cleaning process.

It is understood that the desirable PH value of cleaning solution 58 isrelated to the material of contact plug 56, whose surface is to becleaned. Different materials react differently to different types ofclean solutions. Accordingly, in conventional processes, strong baseswere used for the cleaning of copper after the respective CMP, whileacid or neutral liquids were used for the cleaning of tungsten after therespective CMP. Tungsten, however, may be corroded/etched by acidicsolutions. Also, even if a cleaning solution is neutral, which means itsPH value is 7.0, the galvanic reaction occurs, and tungsten will also becorroded during the cleaning. In the conventional cleaning processes,acid and/or neutral cleaning solutions were used to clean tungsten plugsfor some of their properties in response to tungsten. Acid and/orneutral cleaning solutions, however, also result in the corrosion oftungsten plugs. In conventional structures, the loss of tungsten causedby the cleaning did not post a problem. The reason is that the tungstenplugs in conventional structures were relatively wide, and the depths ofthe resulting recess caused by cleaning were relatively small comparedto the widths of the tungsten plugs. Accordingly, the subsequentlyformed overlying conductive materials may easily fill the recess withoutcausing open circuit. In newer generation of integrated circuits,however, the widths of tungsten plugs are very small, for example, mayreach about 20 nm or smaller. The corrosion depths caused in theconvention cleaning thus become comparable to, and sometimes greaterthan, the widths of contact plug 56, resulting in deep and narrowrecesses. When overlying conductive features 72 (FIG. 12) such ascontact plugs, vias, metal lines, etc. are formed, it is thus difficultto fully fill such a recess, and open circuit may be resulted.

In accordance with some embodiments of the present disclosure, aninhibitor is added into cleaning solution 58 (which contains water) toprevent the corrosion of tungsten plugs. In accordance with someembodiments of the present disclosure, the inhibitor is formed oforganic amine, which may be, for example, C₅H₁₅NO₂. C₅H₁₅NO₂ has amolecular weight equal to 121.18. The inhibitor such as the organicamine causes the PH value in cleaning solution 58 to be basic/alkaline(with PH value higher than 7.0). This is different from conventionalcleaning solutions used after the CMP of tungsten. The inhibitor incleaning solution 58 is attached to the exposed surface of contact plug56, wherein layer 62 is drawn to represent the attached inhibitor. Theinhibitor is negatively charged. Layer 62 is a carbon-concentratedlayer, and layer 62 protects the underlying contact plug from therecessing of the cleaning solution 58. As a result, the attachedinhibitor 60 electrically cuts the path between cleaning solution 58 andcontact plug 56, and electrons are prevented from being flowing intosource/drain region 22, and positive tungsten ions are prevented fromdissolving into cleaning solution 58. The corrosion is thus reduced.

The inhibitor in cleaning solution 58 functions well when the cleaningsolution 58 is a weak base solution with PH values in the range between(not including) 7.0 and about 8.0. Alternatively stated, the PH value isgreater than 7.0 and lower than about 8.0. In neutral or acidicenvironment, the inhibitor is unable to attach to the surface of contactplug 56, and corrosion will occur. On the other hand, if the PH value ofcleaning solution 58 is increased to greater than about 8.0 or higher,the inhibitor also starts losing its inhibiting function increasingly,and tungsten will be corroded/etched by the respective strong basesolutions. In accordance with some embodiments of the presentdisclosure, cleaning solution is adjusted to greater than 7.0 and lowerthan about 8.0 to allow the inhibitor to function.

In accordance with some embodiments of the present disclosure, theadjustment of the PH value of cleaning solution 58 is achieved byadjusting the concentration of the organic amine. For example, theconcentration of the organic amine in cleaning solution 58 may beadjusted to between about 0.5 percent and about 2 percent. A bufferagent such as (hydroxymethyl) aminomethane (TRIS) or saline sodiumcitrate (SSC) may be added into cleaning solution 58, making cleaningsolution 58 a buffer solution, so that its PH value may remain constantwithin the desirable range (between 7.0 and about 8.0). In accordancewith some embodiments, other chemicals such as HCl may be added toadjust the PH value of cleaning solution 58.

After the cleaning of wafer 10 using cleaning solution 58, the residueon wafer 10 is spun off and brushed off. Clean air 64 may be blown onwafer 10 to dry wafer 10, as shown in FIG. 9. The respective step isillustrated as step 216 in the process flow shown in FIG. 15. During theperiod of time starting from the time point the CMP is finished to thetime point wafer 10 starts to be dried (or the time point wafer 10 isfully dried), no neutral cleaning solution or acidic solution is used toclean wafer 10. In addition, no strong base solution with PH valuesignificantly higher than 8.0 (such as 8.5) is used. As a result, thecorrosion of contact plug 56 is minimized. FIG. 9 illustrates aresulting wafer 10, with recess 66 (if formed by the cleaning step)being illustrated schematically. By performing the cleaning using themethods in accordance with the embodiments of the present disclosure,recess depth D1 of recess 66 may be smaller than about 50 Å due to thereduced corrosion. If contact plug 56 has a portion protruding over thetop surfaces of ILD 36 before the cleaning, the top surface of contactplug 56 may also be coplanar with the top surface of ILD 36 or higherthan the top surface of ILD 36 after the cleaning.

In the steps as shown in FIGS. 1 through 9, transistor 300 is formed.Referring to FIG. 10, etch stop layer 68 is formed in accordance withsome embodiments of the present disclosure, followed by the formation ofdielectric layer 70. In accordance with some embodiments, dielectriclayer 70 is an inter-layer dielectric, and hence is alternately referredto as ILD1 70. Etch stop layer 68 may also be omitted in accordance withsome embodiments. Accordingly, etch stop layer 68 is illustrated usingdashed lines to indicate it may or may not be formed. Etch stop layer 68may be formed of silicon carbide, silicon oxynitride, siliconcarbo-nitride, combinations thereof, or composite layers thereof. Etchstop layer 68 may be formed using a deposition method such as CVD,Plasma Enhanced Chemical Vapor Deposition (PECVD), ALD, or the like.ILD1 70 may include a material selected from PSG, BSG, BPSG,Fluorine-doped Silicon Glass (FSG), or TEOS. ILD1 70 may also be formedof a non-porous low-k dielectric material, which may be acarbon-containing dielectric material. ILD1 70 may be formed using spincoating, FCVD, or the like, or may be formed using a deposition methodsuch as CVD, PECVD, Low Pressure Chemical Vapor Deposition (LPCVD), orthe like.

FIG. 10 further illustrates the formation of conductive feature 72. Theformation of layers 68 and 70 and conductive feature 72 is illustratedas step 218 in the process flow shown in FIG. 15. In accordance withsome embodiments of the present disclosure, conductive feature 72 is acontact plug, and the etch stop layer 68 as shown in FIG. 10 is notformed. In accordance with alternative embodiments, conductive feature72 is a copper via or a copper line, and etch stop layer 68 is formed inaccordance with these embodiments.

The formation of conductive feature 72 may include forming an opening indielectric layers 68 and 70 to expose contact plug 56, filling aconductive material(s) in the opening, and performing a planarization.Conductive feature 72 may include conductive adhesion/barrier layers 74,and metallic material 76 over adhesion/barrier layers 74.Adhesion/barrier layer 74 may be formed of a material selected fromtitanium, titanium nitride, tantalum, tantalum nitride, combinationsthereof, or multi-layers thereof. Metallic material 76 may be formed oftungsten, copper, aluminum, or alloys thereof, and may be formed usingPVD, Metal-Organic Chemical Vapor Deposition (MOCVD) or plating.Advantageously, if recess 66 in FIG. 9 is formed due to the cleaningprocess, since recess 66 is shallow in accordance with the embodimentsof the present disclosure, conductive feature 72 may easily be filledinto recess 66 without generating open circuit.

FIGS. 11 and 12 illustrate the formation of gate contact plugs. Therespective step is illustrated as step 220 in the process flow shown inFIG. 15. An etching process(es) is performed to etch dielectric layer70, etch stop layer 68, ILD 36 and mask layers 38 (FIG. 10), so thatgate contact openings 78 are formed, as shown in FIG. 11. Next, contactopenings 78 are filled with a conductive material(s) to form gatecontact plugs 80, as shown in FIG. 12. In accordance with someembodiments of the present disclosure, gate contact plugs 80 includeconductive adhesion/barrier layers 82, and metallic material 84 overadhesion/barrier layers 82. The adhesion/barrier layer 82 may be formedof a material selected from titanium, titanium nitride, tantalum,tantalum nitride, combinations thereof, or multi-layers thereof.Metallic material 84 may be formed of tungsten, copper, aluminum, oralloys thereof.

FIG. 13 illustrates carbon-rich layer 86 left on some portions ofcontact plug 88, wherein carbon-rich layer 86 includes the attachedinhibitor 62 (FIG. 8) mixed with the overlying and underlying materials.In accordance with some embodiments, contact plug 56′ is on the samedie/wafer as contact plug 56, and is formed simultaneously as contactplug 56. It is noted that although contact plug 56′ is drawn to be closeto gate stack 26, contact plug 56′ may actually be spaced apart fromgate stacks. Contact plug 56′ has no overlying via or contact plugconnecting to it, and carbon-rich layer 86 is formed on contact plug 56′and left in the final structure. Carbon-rich layer 86 may include a mixof the residue organic amine, which is the attached inhibitor 62 in FIG.8. Carbon-rich layer 86 may also include the material of contact plug56′ and/or the material of etch stop layer 68 or dielectric 70 (whenlayer 68 is not formed). Carbon-rich layer 86 has a higher carbonconcentration than the underlying contact plug 56′. The carbonpercentage in carbon-rich layer 86 may also be higher than that in theoverlying dielectric 68 (or 70). In accordance with some embodiments,the atomic percentage of carbon in carbon-rich layer 86 is higher thanthe atomic percent of carbon in contact plugs 56′ and overlying layer 68(or layer 70 if layer 68 is not formed) by 15 percent or higher. It isrealized that although ILD 36 is also exposed to cleaning solution 58(FIG. 8) during the cleaning, since tungsten tends to carry positivecharges, and the inhibitor in the cleaning solution 58 and ILD 36 tendsto be negatively charged, the inhibitor tends to accumulate on the topsurface of contact plugs 56′ and 56, but not on the top surface of ILD36. Accordingly, in the structure as shown in FIG. 13, there is acarbon-rich layer 86 on the top surface of contact plug 56′, but not onthe top surface of ILD 36.

The carbon-rich layer 86 may exist on all top surfaces of contact plugs56′ and 56 at a time immediately after the cleaning process. As long asthese surfaces are not exposed again after the formation of layers68/70, the corresponding carbon-rich layer will be preserved. Forexample, if the conductive feature 72 is narrower than the underlyingcontact plug 56, in the formation of the opening for filling conductivefeature 72, some top surface portions of contact plug 56 may not beexposed. Accordingly, these surface portions of contact plug 56 willalso have carbon-rich layers over and contacting them.

FIG. 14 illustrates the cleaning of conductive feature 90, which isformed on the same wafer as contact plug 56. Conductive feature 90 maybe formed of a similar metal as contact plug 56, and may be formed oftungsten. In accordance with some embodiments, conductive feature 90 isa gate electrode (such as 28 in FIG. 1), a contact plug, a metal pad, ora redistribution line. Conductive feature 90 has width W3 greater thanthe width W2 (FIG. 8 of contact plug 56. The formation of conductivefeature 90 includes forming an opening (filled by conductive feature 90)in dielectric layer 95, filling a metallic material 94, which mayinclude tungsten, and performing a CMP. After the CMP, a cleaning isperformed using brush 61′ and cleaning solution 58′. In accordance withsome embodiments of the present disclosure, cleaning solution 58′comprises an organic acid, which may include a citrus acid (C₆H₈O₇). Thecleaning may also be performed using a neutral liquid (also representedby 58′) such as de-ionized water. In accordance with some embodiments,the recess 92 formed due to the corrosion has depth D2 greater thanabout 100 Å. Although this depth has a greater value than depth D1 (FIG.9), since width W3 of conductive feature 90 is much greater than widthW2 (for example, ration W3/W2 may be 2.0 or greater), the gap filling ofrecess 92 has no problem, and no open circuit will be generated.Accordingly, in accordance with the present disclosure, the cleaningsolutions may be selected according to the widths of the respectivecleaned features. For example, when forming the wafer, a threshold widthis pre-determined, for example, through experiment based on whether therecesses 66 (FIG. 9) can be filled without generating defect or not. Thethreshold width may be 50 nm, for example, and is determined by aplurality of factors. In the cleaning of all tungsten-containingfeatures having widths greater than the threshold width, acidicsolutions and/or neutral liquids/solutions are used, while in thecleaning of all tungsten-containing features having widths equal to orsmaller than the threshold width, weak base solutions as discussed aboveare used.

The embodiments of the present disclosure have some advantageousfeatures. By adopting an inhibitor in the cleaning solution for cleaningthe wafer, the contact plug is protected from the corrosion caused bythe cleaning solution, and the contact loss is reduced. Furthermore, byadjusting the PH values of the inhibitor, the respective chemical (suchas the organic amine) may function as an inhibitor during the cleaningperformed after CMP.

In accordance with some embodiments of the present disclosure, a methodincludes forming a first dielectric layer over a wafer, etching thefirst dielectric layer to form an opening, filling a tungsten-containingmaterial into the opening, and performing a CMP on the wafer. After theCMP, a cleaning is performed on the wafer using a weak base solution.

In accordance with some embodiments of the present disclosure, a methodincludes forming an ILD with a portion at a same level as a gate stackof a transistor, The ILD and the gate stack are parts of a wafer. TheILD is etched to form a source/drain contact opening. A source/drainregion of the transistor is exposed through the source/drain contactopening. A tungsten-containing material is deposited on the wafer,wherein the tungsten-containing material has a portion filling thesource/drain contact opening. A CMP is performed on the wafer to removeexcess portions of the tungsten-containing material. The wafer is thencleaned using a cleaning solution including organic amine, wherein thecleaning solution is a weak base solution. The wafer is then dried.

In accordance with some embodiments of the present disclosure, astructure includes a first dielectric layer, and a metal plug in thefirst dielectric layer. A top surface of the metal plug is substantiallycoplanar with a top surface of the first dielectric layer. A carbon-richlayer is overlying and contacting the metal plug. The carbon-rich layerhas a first carbon concentration higher than a second carbonconcentration of the metal plug. A second dielectric layer is overlyingand contacting the carbon-rich layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a first dielectriclayer over a wafer; etching the first dielectric layer to form a firstopening; filling a tungsten-containing material into the first opening;performing a first Chemical Mechanical Polish (CMP) on the wafer; afterthe CMP, performing a first cleaning on the wafer using a weak basesolution; and forming a second dielectric layer over and contacting thetungsten-containing material, wherein a carbon-rich layer is formedbetween, and is in contact with, the tungsten-containing material andthe second dielectric layer.
 2. The method of claim 1, wherein the weakbase solution comprises an organic amine.
 3. The method of claim 2,wherein the organic amine comprises C₅H₁₅NO₂.
 4. The method of claim 2,wherein a PH value of the weak base solution is in a range between 7.0and about 8.0.
 5. The method of claim 1 further comprising forming asource/drain region, wherein a portion of the tungsten-containingmaterial left after the CMP acts as a contact plug, and the contact plugis electrically coupled to the source/drain region.
 6. The method ofclaim 1 further comprising: forming a third dielectric layer over thefirst dielectric layer; etching the third dielectric layer to form asecond opening, wherein the second opening is wider than the firstopening; filling a second tungsten-containing material into the secondopening; performing a second CMP on the wafer; and after the second CMP,performing a second cleaning on the wafer using an acidic solution or aneutral liquid.
 7. The method of claim 1, wherein the carbon-rich layerdoes not extend directly on the first dielectric layer.
 8. A methodcomprising: forming an Inter-layer Dielectric (ILD) with a portion at asame level as a gate stack of a transistor, wherein the ILD and the gatestack are parts of a wafer; etching the ILD to form a source/draincontact opening, wherein a source/drain region of the transistor isexposed through the source/drain contact opening; depositing a firsttungsten-containing material on the wafer, wherein the firsttungsten-containing material comprises a portion filling thesource/drain contact opening; performing a Chemical Mechanical Polish(CMP) on the wafer to remove excess portions of the firsttungsten-containing material; cleaning the wafer using a cleaningsolution comprising organic amine, wherein the cleaning solution is aweak base solution having a PH value in a range greater than 7.0 andsmaller than about 8.0; drying the wafer; and forming a dielectric layerover and contacting the first tungsten-containing material, wherein acarbon-rich layer is formed between, and is in contact with, the firsttungsten-containing material and the dielectric layer.
 9. The method ofclaim 8, wherein from a first time point the CMP is finished to a secondtime point the wafer is fully dried, no neutral liquid is used forcleaning the wafer.
 10. The method of claim 9, wherein from a first timepoint the CMP is finished to a second time point the wafer is fullydried, no acid solution is used for cleaning the wafer.
 11. The methodof claim 8, wherein the organic amine comprises C₅H₁₅NO₂.
 12. The methodof claim 8 further comprising: filling a second tungsten-containingmaterial into an additional opening in the wafer, wherein the additionopening is wider than the source/drain contact opening; performing anadditional CMP on the wafer to remove excess portions of the secondtungsten-containing material outside the additional opening; and afterthe additional CMP, cleaning the wafer using an acidic solution or aneutral liquid.
 13. The method of claim 8 further comprising adding abuffer agent into the cleaning solution to stabilize a PH value in thecleaning solution.
 14. The method of claim 8, wherein the carbon-richlayer does not extend onto top surfaces of the dielectric layer.
 15. Themethod of claim 12, wherein the cleaning the wafer after the additionalCMP is performed using a neutral liquid.
 16. A method comprising:forming a source/drain contact opening in a dielectric layer of a wafer;filling the source/drain contact opening with tungsten; performing aChemical Mechanical polish (CMP) to remove excess portions of thetungsten, wherein a remaining portion of the tungsten forms a portion ofa source/drain contact plug; cleaning the wafer using a cleaningsolution comprising C₅H₁₅NO₂; and drying the wafer, wherein from a firsttime point the CMP is finished to a second time point the wafer isdried, all solutions used for cleaning the wafer have PH values in arange between 7.0 and about 8.0, and wherein after the drying the wafer,a carbon-rich layer exists at a top surface of the source/drain contactplug.
 17. The method of claim 16, wherein the cleaning solutioncomprises a buffer solution and water.
 18. The method of claim 16,wherein at a time the drying the wafer is started, a recess isgenerated, with the recess extending from a top surface of thedielectric layer to a top surface of the source/drain contact plug, andthe recess has a depth smaller than about 50 Å.
 19. The method of claim16, wherein the CMP is performed with the cleaning solution beingsprayed on the wafer for a duration between about 1.5 minutes and about2.5 minutes.
 20. The method of claim 16, wherein the carbon-rich layerdoes not extend directly on a top surface of the dielectric layer.